The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities. This definition includes implantable devices for continuous monitoring of the heart rhythm and to deliver if necessary electrical stimulation, defibrillation or resynchronization pulses to the heart. It also includes neurological devices, cochlear implants, medical drug pumps, implantable biological sensors, etc.
These devices include a housing generally designated as the generator electrically and mechanically connected to one or more intracorporeal leads provided with electrodes for contacting the tissue in which it is desired to apply stimulation pulses and/or to collect an electrical signal (myocardium, nerve, muscle, etc.).
The present invention more precisely relates to a detection/stimulation microlead intended to be implanted in the venous, arterial, or lymphatic networks. Performance of the stimulation of a heart chamber by an implantable lead in the coronary network will more specifically be described in the present application, but this application is not restrictive, and the microlead of the invention can be used in many other configurations and applications permitted by its very small diameter.
In this example of coronary leads for stimulating a left, atrial or ventricular, cavity of the heart, the lead is not inserted into the cavity to stimulate but in the coronary network, and is provided with an electrode for contacting the wall to the epicardium at the level of the left ventricle or of the left atrium, as appropriate. These leads thus stimulate the heart muscle via one or more point electrodes whose position is a function of the predefined trajectory of the cannulated vein.
With the lead not being placed within the cavity but against a wall, the importance of a correct orientation of the electric field generated by the electrode must be understood, so as to ensure the orientation of the electric field to the cardiac muscle through the vein wall, to reduce the pacing threshold and, consequently, the energy required for the stimulation.
With conventional leads, e.g. the Situs LV model sold by Sorin CRM, Clamart, France, and which is described in particular in EP 0993840 A1 (ELA Medical), the lead stimulates the cavity through an annular ring electrode blocked in the vein and contacting on the whole periphery thereof in the region of the stimulation site. The electric field is then distributed in all directions on 360°, which corresponds to an “annular” radiation. A part of the radiation will necessarily be oriented in the direction of the heart muscle due to the constant contact with the vein, which guarantees delivery of the stimulation energy of the cavity where the electrode has been implanted.
However, a significant portion of the electric field is not optimally used, as it is directed opposite to the heart muscle, which corresponds to an unnecessary energy for stimulation. Indeed, most of the electric field is distributed in the blood, whose electrical conductivity is higher than that of the muscle tissue, thus resulting in poor performance for electrical cardiac pacing.
WO 02/180006 A2 describes a lead of a similar type, wherein the distal end includes an included helical preshape, the diameter of which is between 2.5 and 20 mm, with regularly distributed electrodes at 120° on this preshape. The configuration of this preshape notably allows an effective and stable contact at the interface between the lead and the wall of great diameter vessels, the helical preshape mechanically pressing, by elasticity, the lead against the inner wall of the vessel.
A recent trend in the implantable stimulation lead in venous, arterial, or lymphatic network is the reduction in diameter, typically to a diameter of less than 2 French (0.66 mm), or reaching 0.5 French (0.17 mm). This is much lower than that of conventional leads such as the Situs LV model described above, the diameter of the active portion of which is of the order of 4 to 7 French (1.33 mm to 2.33 mm) or the leads such as those described in the WO 02/180006 A2 cited above.
The size of the lead body is indeed a factor directly related to the capabilities of controlled guiding of the lead, for example in the coronary venous network, which allows to select specific stimulation sites located in certain collateral veins. The very small diameter outside the active distal end of the lead thus allows cannulation of very thin veins of the coronary network, so far not used because of the excessive size of conventional coronary leads.
Such leads, which can be described as “microleads” are described for example in EP 2455131 A1, EP 2559453 A1 and EP 2581107 A1, all three on behalf of Sorin CRM SAS. The active portion of these microleads is constituted by a microcable having a diameter of about 0.5 to 2 French (0.17 to 0.66 mm) having a plurality of exposed portions forming a succession of individual electrodes constituting together a network of electrodes connected in series to multiply the points of stimulation in a deep area of the coronary network.
As described in particular in EP 2559453 A1 above, the very small diameter of the microcable can allow for introduction in a first vein (“go” vein), then by an anastomosis to a second vein (“return” vein) ascending therein. A very frequent presence of distal anastomosis in the coronary venous network has been found, that is to say that there is at the end of certain veins a passage to another vein, therefore with a possibility of communication between two separate veins at the anastomosis, via their respective distal ends. This makes it possible, with a single lead, to simultaneously stimulate two relatively remote areas, because they are located in two separate veins. The double effect of the distance of these two areas and of the multiplication of points of stimulation in each area provides a particularly beneficial effect on the resynchronization of the functioning of the heart.
Another advantage of the small diameter of the active part of the lead is that it avoids the blockage of a part of the blood flow in the vein, which would result in a deficiency of the venous network irrigation downstream of the lead tip.
Reducing the diameter of the lead is nevertheless not devoid of drawbacks. Indeed, when the diameter of the lead is substantially lower than that of the vein, it is difficult to assure the permanent contact of the electrodes. The exposed portion of the microcable which forms an electrode may thus be in an intermediate, “floating”, position in the middle of the vein, the contact points between the microcable and the vein wall occurring on electrically isolated areas.
This is particularly true in the case of microcables passing through an anastomosis. Indeed, if the veins are of small diameter, typically less than 1 French (0.33 mm) in the region of the anastomosis, beyond the anastomosis they may join the coronary sinus after passing the left ventricle, and in this case their diameter increases. The very thin microcable which has allowed passing the anastomosis may then move into a region of relatively large diameter, thereby with a difficulty in establishing a stable contact between the electrodes and the vein wall in the area.
This drawback (no guaranteed contact) is the counterpart to the advantage mentioned above to avoid the obstruction of blood flow in the vein. In contrast, the larger diameter leads including an annular stimulation ring blocked over the entire periphery of the vessel ensure contact with the target tissue, but necessarily involve an obliteration of this vessel which can have deleterious effects. Furthermore, from the electrical point of view, it is important to ensure, firstly, the effectiveness of the stimulation despite a vessel having a diameter larger than that of the stimulation microcable and, secondly, optimized power consumption even with a configuration of only three electrodes oriented at 120° applicable in anastomoses.